Retortable pouches are required to have not only those performance characteristics that are required of food packaging materials but also resistance to retort treatment. Generally, this retort treatment is conducted under pressure at a temperature of 110.degree. to 120.degree. C. for about 30 to 60 minutes. Meanwhile, nylon films have an advantage over other plastic films in that they have good wear resistance and good optical properties such as gloss and transparency, among others, and particularly in that they have good oil resistance, heat resistance and cold resistance, hence can be used in a wide temperature range. Therefore they are widely used as packaging materials for foods to be retort-treated. However, nylon films are still insufficient in gas barrier properties such as oxygen gas and steam barrier properties.
For increasing gas barrier properties, it is of use to apply a vinylidene chloride polymer to plastic films. Thus, for instance, Japanese Patent laid open No. 233836/1988 proposes gas barrier films comprising a plastic film and a layer containing a swellable silicate having a layer structure, for example montmorillonite, and a vinylidene chloride polymer as formed on at least one side of the film. Generally, however, films in which a nylon film is used as the plastic film are inferior in heat sealability and, furthermore, their gas barrier properties markedly deteriorate upon retort treatment.
Therefore, for providing composite materials while providing them with gas barrier properties, laminated films have also been proposed which comprise a nylon film, a vinylidene chloride polymer film laminated thereon, a laminating agent containing a polyisocyanate component and a polyol component as applied to the latter film and, further, a film laminated thereon.
The laminated films having the constitution mentioned above show high gas barrier properties before retort treatment but, upon retort treatment, their gas barrier properties decrease markedly. In addition, the laminated films become hazy upon retort treatment, which results in markedly decreased transparency. Furthermore, pouches or the like made from such films by heat sealing or the like, when subjected to retort treatment, undergo marked whitening in the sealed portions. The extent of this whitening increases with the increasing heat sealing temperature. The causes of these are presumably as follows. Thus, the isocyanate component in the laminating agent reacts with moisture in air or, in some instances, with an active hydrogen atom of the vinylidene chloride polymer contained in the coating layer mentioned above. Since this causes partial deactivation of the isocyanate component, the unreacted polyol component remains in excess in the laminating agent layer. Upon retort treatment, the unreacted polyol component that remains migrates into the coating layer mentioned above and inhibits crystallization of the vinylidene chloride polymer, whereby the transparency and gas barrier properties of the laminated films are impaired. Further, it is presumable that when pouches or the like are manufactured by heat sealing, the sealing heat causes melting of the vinylidene chloride polymer to thereby destroy the crystalline structure; the heat-sealed portions thus may readily undergo whitening when exposed to high temperature water for retort treatment.